This invention relates to noise suppression in communications systems and, more particularly, to the suppression of image noise in superheterodyne communications receivers using an on-chip image rejection filter.
Superheterodyne receivers continue to be used as the basic architectural element in mobile communications systems. A superheterodyne receiver front-end typically consists of a low-noise amplifier (LNA), an image reject filter, a mixer, and a VCO as shown in FIG. 1. An LNA with very low noise figure is typically required to enable the receiver to detect very weak signals. Additionally, the LNA must provide sufficient gain to suppress the noise generated by the stages that follow it. The mixer allows for downconversion or the translation of the desired signal from the radio frequency (RF) to an intermediate frequency (IF) for further processing by the receiver backend.
A well-known occurrence in superheterodyne receivers is that the front-end low-noise amplifier (LNA) may generate thermal noise at an image frequency (located a distance of two IFs away from the desired radio frequency, and that during downconversion, the image noise will fold over onto the thermal noise at the desired receiver frequency. In a more likely scenario, however, some other signal broadcasted at the image frequency may be received by the antenna and amplified by the LNA along with the desired signal. In any case, some form of image noise rejection is required prior to downconversion to suppress the unwanted image signals.
Currently, xe2x80x9coff-chipxe2x80x9d passive filters, such as surface acoustic wave (SAW) filters or ceramic filters are often used for image rejection. These filters, however, represent a major impediment to raising the level of integration of wireless radios, since they cannot be easily integrated with the silicon (Si) substrate. Furthermore, their use complicates the design of the receiver front-end. For example, the LNA must be designed to drive 50 ohms as its output comes off-chip. Matching from the LNA to the filter must, therefore, be performed either on-chip or on the board. The filter must then be matched to the mixer input, which must also be driven by 50 ohms. This implies that large radio frequency (RF) signals will be present on two bond wires in the package used for the receiver. And since bond wires are a major source of signal coupling, this leads to increased signal coupling between respective ports in the receiver. Furthermore, additional pins must be included to accommodate the off-chip filter leading to more expensive packages and, often, a larger integrated circuit (IC) die.
Off-chip filters also represent a significant fraction of the overall cost of the receiver front-end. However, if effort is expended to integrate the filter monolithically, then the signal will never have to leave the chip before reaching the IF stage in the receiver chain. In this way, a simpler cheaper package may be used and the costly off-chip filter can be eliminated.
To date, monolithic image rejection has only been demonstrated satisfactorily using image reject mixers. However, such implementations have not provided the level of rejection required to meet the specifications of existing cellular standards. A monolithic image reject filter integrated between the LNA and the mixer continues, therefore, to be a challenge for the RF design community.
An obvious approach to the realization of a monolithic image reject filter is to use a bandpass filter to pass the desired signal and not the image. However, a monolithic bandpass filter suitable in performance for a receiver front-end has not yet been demonstrated. Low-power tunable monolithic bandpass filters with high selectivity are difficult to realize, due to stability, linearity and noise considerations.
Recently, a few xe2x80x9con-chipxe2x80x9d notch filter implementations of image reject filters have been reported. However, the majority of these suffer from poor linearity and poor noise figure (i.e. they become limiting factors in the quality of the information being received). Most of these implementations also require a great deal of extra circuitry and chip area, not to mention current.
As the popularity in wireless personal communication systems (PCS) continues to grow, the need for low-power, low-cost radio receivers is paramount. Low-cost can be achieved by integrating the required functions as much as possible, and minimizing the number of off-chip components.
Previous on-chip solutions for integrating the image rejection filter on a receiver front-end have employed an additional stage following the low-noise amplifier (LNA) to perform the image rejection function for the receiver. The present invention, however, discloses a novel topology for integrating an image reject filter with a traditional LNA for use in the front-end of a superheterodyne receiver.
According to a first broad aspect of the invention, the conventional topology for an LNA is modified by replacing the degeneration inductor with a resonator to provide a notching action in the frequency response of the LNA at the unwanted image frequency component. The LC resonator is centered at the image frequency and presents a high impedance to the emitter of the driving amplifier so as to cause the driving amplifier to have a substantially low gain at the image frequency.
According to a second broad aspect of the invention, a multi-terminal circuit element having a first conducting terminal for providing an output signal, a second conducting terminal connected to an inductive degeneration element and a control terminal for receiving an input signal is AC coupled via its second conducting terminal to a filtering network. The filtering network is adjusted to provide a substantially high impedance to the second conducting terminal of the multi-terminal circuit element at the image frequency of the input signal so as to cause an unwanted image frequency component to be substantially eliminated in the output signal of the multi-terminal circuit element.
The topology of the present invention requires minimal additional circuitry to perform the filtering function, uses only minimal additional current and does not suffer from the same performance limitations as currently used topologies.